How to Set Transparency to One Specific Color in EEVEE
Oh my actually it's simple, I use wrong shader. It should be principled BSDF, not emission. So what actually to do is:
 - Set black color of color ramp to alpha. Decrease alpha value to zero should work
 - Color output of color ramp must be connected to color of Principled BSDF, and alpha output to alpha input.